Video content such as that seen on a television screen often has a relatively low resolution. Because multiple images are being displayed in rapid succession, the viewer tends not to notice the relatively poor quality of each image. The lower resolution of the video is tolerated so that the amount of information in the video stream can be reduced. In the case where video is communicated across a communication channel having a reduced information carrying capacity, the information flowing through the channel cannot exceed the capacity of the channel. Using lower resolution video that is not perceived by a viewer to be of low quality is therefore an advantage in certain communication situations. Similarly, it may be desired to be able to store the total information content of a sequence of images on a storage medium of limited storage capacity. Being able to use a lower resolution without the viewer noticing the lower resolution in the regenerated video allows the total amount of information stored to be reduced. More video can therefore be stored on the storage medium. Using lower resolution video is therefore an advantage in certain storage situations.
If, however, such a lower resolution video stream is frozen such that a component image is displayed as a still image, then the viewer may recognize the lower resolution of the image. The image may appear to be of noticeably poor quality. This is undesirable.
U.S. Pat. No. 5,987,179 describes a method for encoding a high-fidelity still image in an MPEG bit stream. An original image is selected for encoding as a high-fidelity still image. The original image is encoded as an I-frame. If the I-frame were decompressed and displayed, then the recovered image would not be the same as the original image. Some image information is lost due to the MPEG encoding process. In the method of U.S. Pat. No. 5,987,179, an enhancement frame is added to the bit stream. The enhancement frame includes the encoded difference information between the original image and the previous encoding of the original image. The decoder decodes the I-frame and uses the difference information in the enhancement frame to improve the quality of the regenerated I-frame image. The resulting improved image is of the same resolution as the other uncompressed I-frames of the uncompressed stream and therefore is a rather small still image.
The MPEG2 protocol specifies a “scalability” technique involving multiple layers. The base layer provides the basic spatial resolution. An enhancement layer uses the spatially interpolated layer to increase the basic spatial resolution. While it is possible to transmit a higher resolution video stream using this scalability. technique, the entire video stream is increased in resolution. The resolution of a particular image of interest is therefore generally restricted due to overall communication channel bandwidth limitations and. increasing the resolution of all the images of the video stream.
U.S. Pat. No. 5,988,863 describes another method wherein a resolution enhancement layer is generated in addition to an original MPEG2 stream. In one embodiment, the enhancement layer is created by expanding the decoded base layer, taking the difference between the original image and the decoded base layer, and compressing. Like the underlying base MPEG2 stream, the enhancement layer includes I-frames, B-frames and P-frames and employs complex MPEG2-like motion compensated prediction. The resulting pair of streams therefore consumes an undesirably large amount of bandwidth to transmit.
An improved method of embedding a high resolution still image in a lower resolution video stream is therefore desired.